Over the last decade, cancer of the prostate has become the most commonly diagnosed malignancy among men and the second leading cause of male cancer deaths in the western population, following lung cancer.
Early detection and treatment of prostate cancer before it has spread from the prostate gland, reduces the mortality of the disease. This is particularly true for younger men who are at greater risk of dying from this pernicious but slowly growing malignancy. This realization has prompted increasing efforts for early diagnosis and treatment. Indeed, the American Cancer Society and the American Urological Association recommend that male population at large undergo annual screening for prostate cancer beginning at age 50. The recommended age for screening is lowered to 40 for men giving a family history of prostate cancer or other risk factors.
With this increasing focus on prostate cancer screening, more men than ever before are being routinely tested for prostate cancer. Not surprisingly, this practice has increased early detection of onset of the disease, as reflected by an apparent increase in the incidence of prostate cancer and decrease in the apparent average age of diagnosis. The clinical hope is that earlier detection of prostate cancer before it metastasizes will reduce the overall mortality rate. Healthcare payers look for early screening and detection to translate into a reduction in the healthcare burden, as early treatment can be less radical, more successful and therefore provided at a lower cost per treated patient. The key to accomplishing this goal remains providing better differential diagnostic tools.
Screening for prostate cancer now involves both palpation of the prostate by digital rectal examination (DRE) and assay of plasma levels of prostate specific antigen (PSA/hK3/hKLK3). PSA is a serine protease produced by the prostatic epithelium that is normally secreted in the seminal fluid to liquefy it. Disruption of the anatomic integrity of the prostate gland can compromise the cellular barriers that normally restrict PSA to within the duct system of the prostate, allowing it to disperse into blood or urine. A number of conditions can result in leakage of PSA into the blood. They include inflammation of the prostate, urinary retention, prostatic infection, benign prostatic hyperplasia (BPH), and prostate cancer. Physical manipulation of the prostate can also increase serum PSA levels, but a mild stimulus, such as digital rectal examination (DRE), does not normally increase serum PSA. It is therefore not surprising that screening of serum PSA as an indicator of prostate cancer is not absolutely predictive.
Despite the fact that measure of blood PSA levels can be the result from a variety of different causes, it is nonetheless the basis for primary screening for prostate cancer. Measurement of total PSA (tPSA) as a diagnostic assay to predict prostate cancer has been in use since 1991. Levels of 4 ng/ml or greater in blood serum are considered abnormal and predictive of prostate cancer. However, the sensitivity of such elevated tPSA levels is only 79%; thus leaving 21% of patients with prostate cancer undetected. The specificity for all tPSA values of 4 ng/ml or greater is very poor. In addition, estimates of specificity for tPSA levels>4.0 ng/ml are reported to be in the range of 20% to 59%, averaging around 33%. The vast majority of false positives are ultimately shown to be benign prostatic hyperplasia (BPH). The specificity is lowest for modestly elevated tPSA, in the low so-called gray zone of 4 to 10 ng/ml. This low level of specificity results in additional more invasive and costly diagnostic procedures, such as transrectal ultrasounds and prostate biopsies. Such tests when unnecessary are also very traumatic for the patient. The psychological impact of being diagnosed as positive until proven as a false positive should not be understated either.
Because of the shortcomings of tPSA, research has been focused on attempting to develop PSA derivatives to increase the sensitivity and specificity of this general diagnostic approach.
One modification is free PSA (fPSA), which was FDA approved in 1998. PSA in serum can be found either in an unbound form or complexed with circulating protease inhibitors, most commonly with alpha-1-antitrypsin (ACT). Clinicians have shown that the proportion of PSA bound to ACT was significantly higher in men with prostate cancer than in unaffected men or those with BPH. As a guideline, if 25% or less of total PSA is free, this is an indicator of possible prostate cancer. The fPSA assay was approved for use in men with tPSA's for 4 to 10 ng/ml. Thus, the fPSA assay was positioned to improve the specificity over that of tPSA alone. However, the predictivity of the fPSA test is not as good in people with really low or really high tPSA levels. Very low tPSA, regardless of measured fPSA, is predictive of not having cancer, while the converse is true with very high tPSA levels. The diagnostic usefulness of fPSA is relatively limited as it can be associated with either BPH or prostate cancer. The use of fPSA in combination with tPSA has been shown to reduce the number of unnecessary biopsies by about 20%.
Clearly, prostate biopsy is the gold standard for confirming prostate cancer. However, even a biopsy is not always 100% certain. The standard is the sextant biopsy where tissue sample collection is guided by transrectal ultrasound. Often six samples are not enough to detect the cancer and either a second biopsy procedure or more than six samples are required.
Despite the improvements in prostate cancer screening over the last ten years, there remains a large unmet need in diagnostic sensitivity and specificity, even when these tools are used in combination. Coupling this need with the large incidence of prostate cancer and the importance for early, accurate detection, the potential usefulness for a true differential diagnostic tool is very significant.
A new prostate cancer marker, PCA3, was discovered a few years ago by differential display analysis intended to highlight genes associated with prostate cancer development. PCA3 is located on chromosome 9 and composed of four exons. It encodes at least four different transcripts which are generated by alternative splicing and polyadenylation. By RT-PCR analysis, PCA3 expression was found to be limited to the prostate and absent in all other tissues tested, including testis, ovary, breast and bladder. Northern blot analysis showed that PCA3 is highly expressed in the vast majority of prostate cancers examined (47 out of 50) whereas no or very low expression is detected in BPH or normal prostate cells from the same patients [Cancer Res 1999 Dec. 1; 59(23):5975-9]. Moreover, a recent study comparing the clinical performance of RNA telomerase RT and RNA PCA3 detection in the case of prostate cancer showed that the PCA3 gene can be considered as a better marker (Cancer Res 2002 May 1; 62(9):2695-8).
The PCA3 gene is composed of 4 exons (e1-e4) and 3 introns (i1-i3). While PCA3 appears to be recognized as the best prostate-cancer marker ever identified, this specificity has been contested in the literature. For example, Gandini et al. 2003, claim that the prostate-specific expression of PCA3 is restricted to that of exon 4 of the PCA3 gene. However, the applicants have shown in a recent patent application that this is not the case (Patent application CA 2,432,365).
In view of the fact that advanced prostate cancer remains a life threatening disease reaching a very significant proportion of the male population, there remains a need to provide the most specific, selective, and rapid prostate cancer detection methods and kits.
The present invention seeks to meet these and other needs.
The present description refers to a number of documents, the content of which is herein incorporated by reference in their entirety.